1. Field of the Invention
The present invention relates to an electron gun assembly for a cathode ray tube (CRT), and more particularly, to an electron gun assembly for a CRT that ensures good alignment between electrodes and that is highly resistant to voltages.
2. Description of the Related Art
A projection display system that utilizes CRTs includes three CRTs for realizing red, green, and blue monochromatic display, and an optical system for enlarging each of the monochromatic images and projecting them onto a projection screen so as to combine the individual images into a single color image.
Unlike the conventional color CRTs that use three electron beams, the projection-type CRT emits a single electron beam to realize images. The image of each of the CRTs is enlarged to approximately ten times its original size before projection onto a projection screen.
Accordingly, in the projection-type CRT, a high current electron beam is used, and there is also a high electric potential between a focus electrode and a final anode electrode such that highly detailed images with a high degree of brightness are realized. Further, the focus electrode and the final anode electrode are fabricated so as to have large apertures in order to increase an aperture of a main focus lens such that greater image detail may be realized at high currents.
The electron gun assembly typically includes a cathode that emits electron beams, first through fifth electrodes for focusing and accelerating the electron beams emitted from the cathode, and a bead glass for fixedly aligning the first through fifth electrodes in a row. Auxiliary supports are fixed to each of the electrodes, and the auxiliary supports are fused to the bead glass.
The first electrode, the second electrode, and the cathode form a triode structure that controls electron emission of the cathode. The third electrode and the fifth electrode, which act as an anode electrode, receive a high anode voltage of approximately 32 kV so as to form a powerful focus lens between the third electrode and the fourth electrode. The fourth electrode, which is a focus electrode, includes a small aperture section and a large aperture section. Part of the small aperture section and all of the large aperture section are positioned within the fifth electrode to thereby increase an aperture of the main focus lens formed between the fourth electrode and the fifth electrode.
With the above electron gun assembly, precision in the formation of each electrode and good alignment of the electrodes are extremely important for realizing a high level of image detail. Since the small aperture section of the fourth electrode contacts an assembly jig during electron gun assembly, it must be precisely formed.
However, there are limits as to how precisely the cylindrical fourth electrode, and in particular, the small aperture section may be formed using present press manufacturing methods. As a result, good alignment of the electrodes during assembly is not realized, ultimately reducing picture quality. Furthermore, manufacture is made complicated by the fact that the large aperture section of the fourth electrode is positioned within the fifth electrode.
The fifth electrode includes a small aperture section and a large aperture section. The fifth electrode is fixed to the bead glass by an auxiliary support which is mounted to an outer circumference of the small aperture section and to an inner circumference of the bead glass.
With this configuration, since a maximum diameter of the combination of the small aperture section of the fourth electrode and its auxiliary support is greater than an inner diameter of the small aperture section of the fifth electrode, difficulties arise during assembly. Therefore, the fourth electrode is typically separated into two sections to perform manufacture. That is, the fourth electrode is divided into a front section and a rear section, and after the fifth electrode and the front section of the fourth electrode are mounted to the bead glass, the rear section of the fourth electrode is moved through the fifth electrode and abutted to the front section. The front section and the rear section are then welded together.
However, productivity is reduced as a result of the many steps involved in this process. Also, the alignment between the front section and the rear section of the fourth electrode is often not exact following welding of these elements. Overall alignment of the electron gun assembly is therefore negatively affected, thereby making it difficult to realize high resolution images.
In a separate issue, because of the large difference in potential between the fourth electrode and the fifth electrode, a sufficient gap must be provided between these elements so that the ability to withstand high voltages is maintained. However, a sufficient gap between the fourth electrode and the fifth electrode is not possible because of structural constraints. In particular, a sufficient gap is not possible because of the particular structure used in an effort to maximize electrode apertures (i.e., the aperture of the main focus lens) by positioning part of the fourth electrode within the fifth electrode, and the fact that this configuration is mounted within a limited aperture of a neck (the portion of the CRT in which the electron gun assembly is mounted). Hence, good voltage resistance characteristics of the conventional electron gun assembly are not realized, thereby increasing the likelihood of defects caused by voltages.